Fiber optic technology is playing an ever-increasing role in the modern age of communications. As communication standards such as Fibre Channel (1062 Mbps) and Gigabit Ethernet (1000 Mbps) place ever-increasing demands on the physical layer infrastructure, optical fiber is being looked to more and more as the transmission medium of choice. Advancements in optoelectronic devices have furthered the desirability of fiber-based arrangements, since these arrangements not only support the necessary high data transmission rates, but the cost is becoming more and more affordable.
A key aspect of the affordability is the modularity by which the advancements in optical fiber technology are being implemented, particularly with regard to their backward compatibility with existing network components, such as multimode fiber. Advancements that cannot be delivered to the marketplace with backward compatibility may not be as desirable as competing advancements which are backward compatible. For example, if an advancement requires recabling an entire building from multimode to single mode fiber, then it may not be a viable solution. If an advancement requires specialized equipment, connectors, patch cords and the like, then it too may not be a viable solution. Accordingly, a desirable feature of any new technological advancement is the modularity and/or compatibility with existing components.
One “legacy” aspect of fiber-based communication systems is their utilization of multimode fiber; that is, a fiber with a relatively large core area that is able to support a plurality of different spatial modes of a propagating optical signal. More recent systems have utilized smaller core, “single mode” fiber, such that when excited by a laser source, only a single spatial mode is supported. In one class of fiber optic communication systems—wavelength division multiplexed (WDM) systems—it is possible to use either multimode or single mode fiber. In particular, multimode fiber can be used with “coarse” WDM (CWDM) which utilizes wavelength spacing of approximately 20 nm. A multiplexer is still required in this system to combine the separate wavelengths into the single transmission fiber.
The extensive earlier deployment of multimode fiber has resulted in the need to, at times, provide coupling between multimode fiber and single mode fiber. Since each spatial mode in the multimode fiber has slightly different propagation characteristics, the coupling of a single mode signal into multimode fiber will inevitably result in spreading the propagating pulses in time (modal dispersion), limiting the useful bandwidth-distance product of the system. Arrangements for improving the bandwidth are considered to be desirable.
In some cases, electronic dispersion compensation (EDC) techniques have been used to correct for the presence of modal dispersion in a received signal. Generally speaking, EDC utilizes arrangements such as active filters to create a set of delayed samples of a received signal (once an optical-to-electronic conversion has been performed). The samples are thereafter scaled and reconstructed to form the original signal. Filtering such as Finite Impulse Response (FIR) or Feed Forward Equalization (FFE) have been found useful for this application. However, the need to include additional electronic components in an optical receiver is not an acceptable solution in some cases, particularly where the physical size and power requirements of the receiver are limited.
It has previously been shown that the bandwidth of a multimode optical fiber can be increased by launching optical signals from a single-mode optical fiber into the multimode optical fiber with a deliberate, predetermined offset between the central axis of the single-mode optical fiber and the central axis of the multimode optical fiber. This feature, referred to as an off-axis or offset launch condition, represents a significant advancement because it has the potential to extend the bandwidth of existing multimode optical fiber installations, such as in a local area network (LAN). By increasing the available bandwidth, the useful life of existing or new installations of multimode fiber may be lengthened. Without the ability to extend the bandwidth, different spatial modes supported by the legacy multimode fiber propagate with different modal group velocities, leading to temporal spreading of a propagating optical signal and thus limiting the speed at which data may be transmitted along this legacy type of fiber.
However, because the dimensions of the offset for an offset launch condition are so small (typically less than 30 μm), the launching single-mode fiber and the receiving multimode optical fiber need to be precisely aligned, preferably within a tolerance on the order of 4-8 μms. Two suggested methods for achieving this precise offset include: the use of a specialized patch cord that incorporates a desired level of offset, or the use of an adapter that precisely aligns the optical fibers so that their cores have a predetermined offset, as described in U.S. Pat. Nos. 6,273,619 and 6,402,390.
While these techniques have some merit, they generally require one or more specialized components or equipment for effectuating an offset launch condition at the fiber interface. Thus, there continues to exist an unsatisfied need in the industry for an optoelectronic module that can be coupled to a multimode optical fiber under an offset launch condition without utilizing specialized equipment or components.